This study investigates the optogenetic induction of long-term depression (LTD) in rat models, focusing on the synaptic connections between the medial geniculate nucleus (MGN) and the lateral amygdala (LA). By utilizing optical fibers and light-sensitive ion channels, researchers can manipulate synaptic strength and assess behavioral outcomes related to cue-associated narcotic-seeking behavior.
Begin with a rat surgically implanted with optical fibers targeting the synapses between neurons of the medial geniculate nucleus (MGN) and the lateral amygdala (LA) .
The rat is conditioned to press a lever to receive a narcotic through a catheter, an action associated with auditory and visual cues.
During repeated conditioning, the auditory signals travel via the MGN and induce upregulation of neurotransmitter receptors at the synapses, strengthening the synaptic connections.
The virally transduced rat expresses light-sensitive ion channels in the MGN neurons.
Deliver low-frequency pulses via the optical fibers to open these channels, a process called optogenetic stimulation.
The resulting cation influx induces low-frequency neurotransmitter release.
The neurotransmitters bind to receptors on LA neurons and induce a low-level calcium influx, which induces neurotransmitter receptor internalization and weakens the synapses.
The weakened MGN-LA synaptic transmission leads to diminished neuronal communication, known as long-term depression (LTD), and reduces cue-associated narcotic-seeking behavior.
For optogenetic induction of long-term depression or LTD, connect patch cables to a 473 nanometer blue laser diode via rotary joint suspended above a clean, standard rodent housing cage. Turn on the laser according to operating instructions and connect the laser to a pulse generator. Adjust settings to give the rat the 900 pulses of light at 2 microseconds at 1 Hertz. Measure the light output through the patch cord using a light sensor, and adjust the laser intensity so that the light output through the patch cable is approximately 5 to 7mw.
Place the rat in the clean housing cage and remove dust covers and ferrule sleeves, exposing the ferrules. Then connect the patch cords bilaterally to each ferrule. If set up properly, rodents will be able to move freely around the cage during optogenetic stimulation. After allowing the rat to explore the environment for three minutes, turn on the pulse generator to initiate optogenetic stimulation. Following long-term depression induction, allow rat to remain in the cage for three minutes before placing it back in the home cage.
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